Synthetic Ceramic Powders Questions and Answers

Answer: C

Correct answer: remains from the Bayer process and harms electrical properties

The Bayer process dissolves bauxite in caustic soda, and a little sodium survives into the calcined alumina. It forms beta-alumina at grain boundaries, which lowers the electrical resistivity and the high-temperature strength - unacceptable in a spark plug insulator or a substrate. Grades are therefore specified as normal, medium or low soda, with the last commanding a premium.

Answer: D

Both are true and the reason gives the necessity: the temperature needed for the phase change is high enough to bond the crystals together, so milling is the only way back to a submicron powder. The milling media then become a source of contamination.

Answer: A

Correct answer: 2 - 4 - 1 - 3

Cost rises with how much processing separates the product from the ground. Mined ball clay is dug, blended and shipped. Acheson silicon carbide is made in an electric furnace from cheap sand and coke, so it is inexpensive despite the energy. Coprecipitated zirconia requires solution chemistry, washing, drying and calcination. And a sol-gel nanopowder adds alkoxide precursors on top of all of that.

Answer: B

Correct answer: heat and mass transfer no longer being uniform in a larger vessel

The chemistry does not change with scale - the transport does. A beaker is stirred and heated almost uniformly; a large vessel has gradients of temperature and concentration, so different parts of the charge experience different conditions and the product becomes non-uniform. Precipitation is especially sensitive, since nucleation depends on local supersaturation, which is precisely what mixing determines.

Answer: C

The driving force is certainly higher, but a nanopowder packs poorly and agglomerates readily, so a badly dispersed one can fire to lower density than a well-packed micron powder. Dispersion decides the outcome as much as size does.